An endoprosthesis for a hip joint normally comprises a ball shaped to fit into a natural or prosthetic acetabulum, a neck extending down and out from this ball, and a stem fitting down into the femur, all made unitarily of a high-strength metallic alloy. The stem tapers downward away from the ball and is received in a normally synthetic-resin sheath whose entire outside surface is solidly cemented into the femur along the full length of the stem. The stem is fixed at least partially to the inside surface of the sheath and may in fact be imbedded in the sheath like the sheath is in the femur.
The sheath serves to compensate for the differences in modulus of elasticity of the metal stem, neck, and ball on the one hand and that of the corticalis and spongiosa of the bone on the other. If the metal part were implanted directly in the bone it would be likely to work relative to the bone and come loose. In addition the major forces the joint would be subjected to would be transmitted by the normally lower distal end of the stem to a distal portion of the femur, something that happens whether or not the neck has a collar sitting on the proximal end of the femur.
In order to obtain a homogenous force transfer between the metal part and the sheath, the section of the metal part if made of steel should be reduced relative to that of the synthetic-resin sheath to about 1/67. Such a reduction is practically impossible due to the likelihood of breakage from insufficient dynamic resistance to bending. In order to avoid such a chance of breakage, a substantially thicker stem is employed.
Another problem with the prior-art arrangements in which the main load is transferred by the endoprosthesis to a nonproximal portion of the femur is that the lack of active stimulation of the hard upper end of the femur causes it to atrophy. This atrophy in turn leads to loosening of the proximal end of the endoprosthesis, greatly shortening the service life of the unit and in fact making its replacement quite complex.